Portable battery pack



FIG. 1 is a top perspective view of a portable battery pack according to the present design;

FIG. 2 is a front elevational view of a portable battery pack according to the present design;

FIG. 3 is a rear elevational view of a portable battery pack according to the present design;

FIG. 4 is a right side elevational view of a portable battery pack according to the present design;

FIG. 5 is an enlarged top plan view of a portable battery pack according to the present design; and,

FIG. 6 is an enlarged bottom plan view of a portable battery pack according to the present design. 

The ornamental design for a portable battery pack, as shown and described. 